Is Your Furnace Blowing Cold Air? These Are the Causes and Solutions

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Last updated: 
April 28, 2024

Troubleshooting a Furnace Blowing Cold Air


Your furnace relies on various components working together to provide warmth throughout your home during colder months. When one or more of these components malfunction, it can cause your furnace to blow cold air. According to our research, some of the most common issues that lead to this problem include incorrect thermostat settings, a faulty pilot light, a dirty filter, a blocked condensate line, and leaks or clogs in the ductwork.

Cause #1: Incorrect Thermostat Settings

If your furnace continuously runs or only blows out cool air sometimes, it could be due to your thermostat being set incorrectly. To resolve this issue, turn the thermostat setting to AUTO. When the fan setting is set to ON, it causes the furnace to blow out air even when it's not producing heat, resulting in cold air. Setting the thermostat to AUTO ensures that the furnace only blows out warm air.

Cause #2: Faulty Pilot Light

A faulty pilot light that doesn't come on or stay on can also cause your furnace to blow out cool air. To troubleshoot this issue, first relight the pilot light and check if it remains on. If not, ensure that the furnace is receiving gas by checking the gas flow and the furnace's gas valve switch. A dirty pilot light can obstruct fuel flow, so clean it if necessary. Grime build-up on the burner can also prevent the pilot light from lighting properly, so have your burner cleaned as well. A faulty gas valve or thermocouple can also cause the pilot light to fail, so adjust or replace these components if needed.

Cause #3: Faulty Electronic Ignition

Newer furnaces use an electronic ignition, which can cause problems if not adjusted properly. Inspect the electronic ignition and have it adjusted or replaced if necessary to restore proper function.

Cause #4: Dirty Filter

A dirty filter obstructs airflow over the furnace's heat exchanger, causing the furnace to run longer and eventually overheat. When the furnace gets too hot, it trips the high-temperature limit switch, shutting off the burners to prevent damage to the heat exchanger. However, the blower continues to blow to keep the furnace at a safe level, resulting in cold air. If you notice that the furnace blows out warm air and then cool air, or stops blowing out air altogether after a while, a dirty filter may be causing the furnace to overheat. To resolve this issue, shut off power to the furnace, inspect the filter, and clean or replace it if dirty. Then, reset the system.

Cause #5: Blocked Condensate Line

In newer high-efficiency furnaces, a blocked condensate line can cause water to flood around the unit. This happens when dirt, dust, mold, or ice clogs the line, causing water to back up into the unit and triggering the furnace switch to shut off to prevent water damage. A broken condensate pump can also cause the condensate line to overflow. To unclog the condensate line, follow these steps:

  1. Shut off power to your HVAC system.
  2. Locate the drip pan on the interior air handler unit.
  3. Use towels or a wet-dry vac to remove moisture from the pan.
  4. Wash away any contamination from the drip pan with soap.
  5. Use the wet-dry vac to clear the block from the drain line. If the vacuum does not clear the clog, try running a pliable rubber tube through the line.
  6. Clean the drain line at the entry point by removing the PVC cover and using hot water with mild soap or distilled vinegar. Allow the solution to sit for about 30 minutes, then rinse the line with fresh water, ensuring it flows freely.
  7. If the condensate line is clogged by ice, cover the line with pipe insulation and heat tape to prevent freezing.

Cause #6: Leaks or Clogs in the Ductwork

Cracks, holes, and gaps in the ducts allow cold air from the attic or crawlspace to enter, making it feel like your furnace is blowing out cool air. Dirt and debris accumulated inside the ducts can also hinder airflow. To address this issue, inspect the ducts in the attic, crawlspace, and around the unit for any openings. Have a professional technician repair any leaks or gaps if needed.


A furnace blowing cold air can be caused by various issues with its components, many of which you can address yourself. However, for more serious problems, it's best to contact a skilled technician to inspect and repair your furnace, preventing further damage to your system.

Incorrect Thermostat SettingsSet thermostat to AUTO
Faulty Pilot LightRelight pilot light, check gas flow, clean pilot light and burner, adjust or replace gas valve or thermocouple
Faulty Electronic IgnitionInspect, adjust, or replace electronic ignition
Dirty FilterShut off power, clean or replace filter, reset system
Blocked Condensate LineShut off power, remove moisture from drip pan, clean drip pan and drain line, insulate and heat tape line if frozen
Leaks or Clogs in DuctworkInspect ducts for openings, have technician repair leaks or gaps

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